A resistive random access memory (RRAM) array includes RRAM cells arranged at intersections of word lines and bit lines. A RRAM cell includes an insulating material (e.g., a dielectric) as a resistive element. The resistance of the insulating material increases when current is passed through the insulating material in one direction, and decreases when current is passed through the insulating material in an opposite direction. Accordingly, a RRAM cell can be programmed to (i) a high resistance state by passing current through the RRAM cell in one direction, and (ii) a low resistance state by passing current through the RRAM cell in an opposite direction. The high resistance state can be used to denote logic high (binary 1), and the low resistance state can be used to denote logic low (binary 0), or vice versa.
RRAM cells that are programmed to high and low resistance states using currents of opposite polarities are called bipolar RRAM cells. Alternatively, RRAM cells can be programmed to high and low resistance states by passing currents of two different magnitudes in the same direction through the insulating material of the RRAM cells. RRAM cells that are programmed to high and low resistance states using currents of two different magnitudes in the same direction are called unipolar RRAM cells.
Each RRAM cell includes an access device such as a diode or a transistor. The access device is connected in series with the resistive element. Using the access device, the RRAM cells in the RRAM array can be selected and deselected during read and write operations.